Radiation is commonly used in the non-invasive inspection of objects, such as luggage, bags, briefcases and the like, to identify hidden contraband at airports and public buildings. Larger objects, such as cargo containers, are also inspected by radiation scanning. The contraband may include hidden guns, knives, explosive devices and illegal drugs, for example. One common inspection system is a line scanner, where the object to be inspected is passed between a stationary source of radiation, such as X-ray radiation, and a stationary detector. The radiation may be collimated into a fan beam, for example, for line (transmission) scanning. Radiation transmitted through the object is attenuated to varying degrees by material between the radiation source and the detectors. The attenuation of the radiation is a function of the density of the materials through which the radiation beam passes. The attenuated radiation is detected and radiographic images of the contents of the object are generated for inspection. The images show the shape, size, and varying densities of the contents.
The inspection of cargo containers at national borders, seaports, and airports, for example, is a critical problem in national security and trade enforcement. Due to the high rate of arrival of such containers, inspection requires rapid imaging of each container. Standard cargo containers are typically 20-50 feet long (6.1-15.2 meters), 8 feet high (2.4 meters), and 6-9 feet wide (1.8-2.7 meters). Larger air cargo containers, which are used to contain a plurality of pieces of luggage or other cargo to be stored in the body of an airplane, may be up to about 238×96×96 inches (6.0×2.4×2.4 meters), for example. Large collections of objects, such as many pieces of luggage, may also be supported on a pallet. Pallets, which may have supporting side walls, may be of comparable sizes as cargo containers and use of the term cargo container will generally encompass pallets, as well. MeV radiation sources are typically required to generate radiation with sufficient energy to penetrate through standard “cargo containers” and the larger air cargo containers.
It has also been proposed to scan large containers with mobile X-ray imaging systems. For example, U.S. Pat. No. 5,638,420 to Armistead discloses a mobile straddle inspection system, wherein a radiation source generating radiation in the MeV range is routed to one side of the frame and a detector array is mounted on an opposing side of the frame as the radiation source. The frame, which may be self-propelled, moves horizontally across the length of the container during scanning. Radiographic images are generated for analysis by an operator.
U.S. Pat. No. 5,692,028 to Geus et al. discloses an X-ray source mounted on a mobile vehicle and a detector supported by a portal shaped assembly extending from the vehicle. During inspection of an object, which can be another vehicle, the mobile vehicle is driven past the object, such that the object passes through the portal shaped assembly.
U.S. Pat. No. 6,292,533 B1 to Swift, et al. discloses a mobile X-ray inspection system for large objects, such as a cargo container carried by a vehicle, that uses an X-ray generating radiation source with a maximum radiation output of 450 keV. The source is supported on a truck and a pencil beam is generated to vertically scan the vehicle. Detectors, also supported on the truck or a boom extending from the truck, are provided to detect radiation transmitted through and scattered by the contents of the object. In use, a vehicle to be inspected parks alongside the scanning unit on the truck. The source and detectors are moved horizontally by a translation system within the truck to horizontally scan the vehicle.
Fixed inspection systems have also been proposed for inspecting large containers. For example, U.S. Pat. No. 4,430,568 to Yoshida discloses an X-ray system for the inspection of packages, including large shipping containers. A conveyor moves the package or container horizontally between the X-ray source supported on a floor and a detector array.
Similarly, U.S. Pat. No. 4,599,740 to Cable discloses a fixed inspection system, where an X-ray source transmits a continuous beam of radiation across a conveyor along which the containers to be inspected are moved. The container may be moved either continuously or incrementally. The radiation transmitted through a container is detected by a “folded” sensor screen or device having two, perpendicular arms, one extending vertically along a side of the container and the other extending horizontally over the top of a container during inspection. The folded sensor enables the system to have a smaller height than would otherwise be necessary in order to detect radiation transmitted through the entire container.
U.S. Pat. No. 5,917,880 to Bjorkholm discloses an X-ray inspection apparatus that may be used to inspect cargo containers with X-ray radiation of about 8 MeV, collimated into a vertical fan beam to scan a truck carrying the cargo. A first detector array is aligned with the fan beam to detect radiation transmitted through the truck. A second detector array is provided to detect radiation forward scattered through the truck. The truck is moved through the vertical fan beam. Data from both detectors is used to determine the average atomic number of the attenuating material in the truck to identify the material content in the truck. Images indicative of the material content are then prepared. Data provided by the first detector array is also used to form radiographs of the truck.
U.S. Pat. Nos. 6,937,692 and 7,397,891, which are assigned to the assignee of the present invention and are incorporated by reference herein, mount an x-ray radiation source and a radiation detector to rails on respective vehicles. The vehicles may be trailers and the rails may be expandable. A car or truck to be inspected, for example, is positioned between the vehicles and the carriages are moved along the rail to scan the car or truck.
High energy radiation scanning systems for scanning large objects, such as cargo containers, generate high energy in the MeV range, over a broad area. Exposure of the cargo containers and the environment to high doses of radiation could pose a threat to workers and other people in proximity to the radiation scanning system as well as to stowaways in the cargo containers, if present. Large amounts of shielding may be required to prevent leakage or scatter radiation to the environment, adding to the expense and size of the system. Radiation exposure in the environment is a threat in lower energy radiation scanning systems, as well. It could also pose a threat to drivers of vehicles being inspected.
In U.S. Pat. No. 7,991,117 B2, which is assigned to the assignee of the present invention and is incorporated by reference herein, the intensity of a radiation beam is modulated during scanning of an object based on the contents of the object to reduce radiation intensity when it is not needed. The information about the contents of the object may be based on historical records about the object or from the current scanning procedure. The information about the intensity modulation may take place essentially in real time.